In mineral oil or natural gas recovery, mineral oil- and/or natural gas-carrying rock formations are drilled. Typical rock formations comprise sandstone formations and/or carbonate formations. In the case of sandstone formations, the quartz particles are caked together by other materials, including carbonates. Of course, carbonatic formations, too, may have a certain quartz content or silicate content.
Owing to the difference between the pressure in the formation and the pressure in the well, the mineral oil or natural gas flows through fine channels, pores or the like in the formation to the well and is conveyed from there to the surface. The pressure in the formation may be of natural origin or may be artificially maintained, for example, by forcing in water, steam or other liquid or gaseous media through an injection well into the formation.
In order to ensure an economical production rate for mineral oil and natural gas, the permeability of the rock formation must reach a certain degree. Frequently, however, the permeability of the rock formation is too low. On the one hand, the natural permeability may already be too low; on the other hand, however, pores which are sufficiently large per se may become blocked with particles and/or precipitates, for example CaCO3 particles or precipitates, in the course of time. As a result, the permeability of the formation is reduced thus reducing the productivity of oil or gas production.
It is known in the art to increase the permeability of subterranean formations by an acid treatment (also referred to as “acidizing treatment” or “acidizing”). By such an acid treatment new channels or pores can also be created in the formation and furthermore precipitates and/or particles plugging the formation may be dissolved. Further details in this context are described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 6th Edt., 2000 Electronic Release, “Resources of Oil and Gas, 3.4.2. General Production Engineering”.
For example, aqueous solutions of HF, HCl or organic acids may be used for the acidizing treatment.
U.S. Pat. No. 7,638,469 discloses the use of methanesulfonic acid (MSA) for acidizing of carbonatic formations. MSA is a strong acid but is far less corrosive as compared to HCl. This is an important advantage over HCl because it is not avoidable that the acid comes into contact with metals surfaces when injecting it into the formation through a wellbore. U.S. Pat. No. 7,638,469 also discloses the use of additives in addition to MSA, such as amidosulfonic acid H2N—SO3H, water soluble corrosion inhibitors such as propargyl alcohol, butynediol or alkoxylated derivatives therefrom, water soluble polymeric corrosion inhibitors, complexing agents such as EDTA, NTA, HEDTA, MGDA, DTPA, polymers for increasing viscosity, surfactants, foamers, foam breakers, enzymes, oxidizing agents, friction reducers, or paraffin controlling agents.
In course of acidizing, typically an aqueous acid solution is injected into the production well. From the production well the acid solution penetrates through the perforation of the casing into the subterranean formation where the acid reacts with the formation and/or impurities therein thereby increasing its permeability.
The acid may be injected at a pressure sufficient to hydraulically disintegrate the formation for creating fissures, pores and channels (“fracture acidizing”) or the acid may be injected at a pressure not sufficient to disintegrate the formation (“matrix acidizing”). In the latter case, an increased permeability results only from the action of the acid.
Strong acids such as HCl show a very fast reaction with carbonatic rocks. Such a fast reaction has the disadvantage that the acid cannot penetrate deeply into the formation before the acid becomes spent. So, the acid only reacts rapidly with such zones of the formation very close to the wellbore while zones of the formation more distant from the wellbore are no longer affected by the acid.
It has therefore been suggested to retard the reaction rate between the acid and the formation in order to allow the acid to penetrate also into zones of the formation more distant from the wellbore.
It is known in the art to use acid-in-oil emulsions, such as the acid-in-oil emulsions suggested by U.S. Pat. No. 4,140,640; U.S. Pat. No. 5,355,958; U.S. Pat. No. 8,551,926 B2 or US 2013/0126176 A1.
It is furthermore known in the art to pretreat the formation with a composition which renders to formation oil-wet thus retarding the action of the aqueous acid on the formation.
Finally, it is also known in the art to add a retarding surfactant directly to the aqueous acid solution. U.S. Pat. No. 3,319,714 discloses a method of acidizing a calcareous formation with an acid comprising additionally an anionic surface active agent. Preferred surfactants are sulfonates of the general formula RSO3X, wherein R is a C8 to C28 hydrocarbon moiety and X is an alkali metal ion or an ammonium ion. U.S. Pat. No. 3,917,536 and U.S. Pat. No. 3,962,101 disclose an acidizing solution comprising additionally a C8- to C18 primary amine.
US 2012/0222863 A1 discloses to use microcapsules filed with alkanesulfonic acids, in particular MSA for acidizing applications in carbonatic rock formations. MSA is released from such microcapsules in the formation with some delay.
It is furthermore known in the art to use alkanesulfonic acids, in particular MSA in methods of dissolving and/or inhibiting the formation of scale on surfaces, such as disclosed by US 2012/0260938 A1, U.S. Pat. No. 8,648,026 B2 and WO 2013/149923 A1. U.S. Pat. No. 8,648,026 B2 discloses a composition for dissolving and/or inhibiting deposition of scale on a surface of a system, such as a heat-exchanger or a cooling tower, comprising an acidic component comprising an alkanesulfonic acid, a wetting agent comprising a surfactant, and a corrosion inhibitor comprising an amphoteric surfactant. In one embodiment, the amphoteric surfactant may be a β-amino or β-imino propionate.